1. Field of the Invention
The present invention relates to a processing method in which a peripheral reinforcing portion of a wafer (for example, a semiconductor wafer), which is formed around a device region of the wafer having devices (for example, semiconductor chips) formed thereon and is thicker than the device region, is removed from the wafer.
2. Description of the Related Art
In general, semiconductor chips, which are used in various electronic apparatuses, are produced by the following method. That is, grid-like rectangular regions are defined by predetermined division lines on a disc-shaped semiconductor wafer, and electronic circuits are formed on surfaces of the rectangular regions. Next, a rear surface of the wafer is ground so as to be thinned. The wafer is divided along the predetermined division lines. As a result, semiconductor chips are produced. In recent years, electronic apparatuses have been greatly improved in compactness and thinness. In accordance with this, semiconductor chips are required to be even thinner, and semiconductor wafers are required to be thinner than previously.
When a semiconductor wafer is thinned, the rigidity of the semiconductor wafer is decreased. Due to this, it is difficult to handle the semiconductor wafer after the thinning, and cracking easily occurs in the semiconductor wafer. In order to solve these problems caused by the thinning, only the rear surface of circular device region, which has semiconductor chips formed on the surface, is ground so as to be thinned, and a ring-shaped peripheral extra region around the device region is thereby formed as a relatively thick reinforcing portion. In this case, since the rear surface is ground, the thick peripheral reinforcing portion projects on the rear surface, and the entire semiconductor wafer has a recessed shape in cross section. The technique, in which only the peripheral portion is allowed to be thick, has been disclosed in Japanese Unexamined Patent Applications Publication Nos. 2004-281551 and 2005-123425.
In conventional techniques, in order to easily handle a thinned semiconductor wafer, a protective tape was applied to a surface of the semiconductor wafer so as to provide rigidity thereto. However, when a metal film composed of gold is provided by a method (for example, deposition or sputtering) on a rear surface of the semiconductor wafer after thinning, the protective tape has insufficient heat-resistance, so that the processing temperature needs to be lower. Due to this, processing is more time-consuming in this case than in normal cases. In contrast, in the above rigidity improvement technique by formation of a peripheral reinforcing portion, since the rigidity is maintained even when the protective tape is peeled from the semiconductor wafer after the thinning, the above rigidity improvement technique by formation of a peripheral reinforcing portion is superior in that deposition or sputtering can be performed without consideration of heat effects.
In the above manner, the semiconductor wafer is finally divided into plural semiconductor chips. In this case, a typical dividing method for semiconductor wafer is a method in which a semiconductor wafer, which is chucked and held on a vacuum chuck-type table, is cut off by a cutting blade. In this method, a rear surface of the semiconductor wafer is applied to and supported by an adhesive surface of a dicing tape which is provided to a ring-shaped dicing frame, and the dicing tape is chucked on the chuck table. As a result, the semiconductor wafer is supported by the chuck table.
In this case, if the semiconductor wafer has a typical plate shape, the entire rear surface of the semiconductor wafer closely contacts the chuck table, so that the semiconductor wafer is stably held. However, when the semiconductor wafer has the above peripheral reinforcing portion, it is difficult to stably hold the semiconductor wafer. In order to solve this problem, it was conceived that the chuck table could be modified to have a shape to which the recessed rear surface of the semiconductor wafer having the above peripheral reinforcing portion is fitted. However, since this modification of the chuck table is not practical, the above peripheral reinforcing portion of the semiconductor wafer is removed, so that the entire semiconductor wafer is processed to have a plate shape.
In a method for removing a peripheral reinforcing portion, as shown in FIGS. 9 and 10, a semiconductor wafer 1 applied on a dicing tape 501 is held on a rotating chuck table, and a cutting blade 502 for dividing the semiconductor wafer 1 into semiconductor chips cuts a circular boundary between a device region 4 and a peripheral reinforcing portion 5a. Next, for example, the semiconductor wafer 1 is rotated by rotating the chuck table 501, so that the peripheral reinforcing portion 5a is cut off. However, in this method, since a cut width 502a by the cutting blade 502 is wider than an edge thickness of the cutting blade 502 due to difference between an inner circumference and an outer circumference of the cutting blade 502, a peripheral portion of the device region 4 is also subjected to the cutting, so that outer diameter of the device region 4 becomes small. As a result, the number of the obtained semiconductor chip becomes small, that is, the yield of the semiconductor chips becomes small.
In order to solve the above problem, as shown in FIGS. 11 and 12, in another method for removing a peripheral reinforcing portion, while a semiconductor wafer 1 is rotated by rotating a chuck table 503, and a grinding wheel 504 having a rotational shaft parallel to that of the chuck table 503 is rotated, plural grinding stones 505 are pressed on a peripheral reinforcing portion 5a, so that the peripheral reinforcing portion 5a is cut and removed. It was conceived that this method was advantageous since the above problem due to the difference between the inner circumference and the outer circumference of the cutting blade 502 was not caused. However, in this method, it is necessary to control the center distance between the chuck table 503 and the grinding wheel 504 with high precision. That is, it is necessary that outermost circumferential edge of rotation locus of the rotating grinding stones 505 corresponds to a boundary between the device region 4 and the peripheral reinforcing portion 5a of the semiconductor wafer 1. Even if the outermost circumferential edge thereof is positioned slightly insider than the boundary between the device region 4 and the peripheral reinforcing portion 5a on the semiconductor wafer 1, the peripheral portion of the device region 4 is ground. Due to this, semiconductor chips on the cut peripheral portion thereof cannot be used, and the yield of the semiconductor chips is decreased. However, this method does not have a function for recognizing and controlling the position of rotation locus of edges of the grinding stones 505, and the grinding stones 505 are away from the rotational shaft of the grinding wheel 504. As a result, in practice, it is difficult to control grinding points of the grinding stones 505 with high precision and fineness, and grinding of the device region 4 cannot be avoided.
In addition, in this method, a case in which the semiconductor wafer 1 is concentric with the rotation center of the chuck table 503 does not cause a problem. However, in a case in which the semiconductor wafer 1 is not concentric with the rotation center of the chuck table 503, this case often occurring, the semiconductor wafer 1 is eccentric. Due to this, when the entire peripheral reinforcing portion 5a is ground, the grinding stones 505 overlap with a portion of the device region 4, so that this portion cannot be used, and the yield of the semiconductor chips is decreased. As shown in FIG. 12, when the center of the semiconductor wafer 1 rotating eccentrically is the most remote from the grinding stones 505 as shown by a broken line, the only peripheral reinforcing portion 5a is ground by the grinding stones 505. However, when the center of the semiconductor wafer 1 rotating eccentrically is proximate to the grinding stones 505 as shown by a two-dot chain line, the grinding stones 505 overlap with the device region 4, and the device region 4 is ground by the grinding stones 505.